(MIS 4) Arctic Ground Squirrel Middens and a Squirrel-Eye-View of the Mammoth-Steppe
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Quaternary Science Reviews 30 (2011) 2220e2237 Contents lists available at ScienceDirect Quaternary Science Reviews journal homepage: www.elsevier.com/locate/quascirev Early Wisconsinan (MIS 4) Arctic ground squirrel middens and a squirrel-eye-view of the mammoth-steppe Grant D. Zazula a,*, Duane G. Froese b, Scott A. Elias c, Svetlana Kuzmina b, Rolf W. Mathewes d a Yukon Palaeontology Program, Department of Tourism & Culture, Government of Yukon, Box 2703 L2-A, Whitehorse, Yukon, Canada Y1A 2C6 b Department of Earth and Atmospheric Sciences, University of Alberta, 1-26 Earth Sciences Building, Edmonton, Alberta, Canada T6G 2E3 c Geography Department, Royal Holloway, University of London, Egham, Surrey TW200EX, United Kingdom d Department of Biological Sciences, Simon Fraser University, 8888 University Drive, Burnaby, British Columbia, Canada V5A 1S6 article info abstract Article history: Fossil arctic ground squirrel (Spermophilus parryii) middens were recovered from ice-rich loess sedi- Received 24 June 2009 ments in association with Sheep Creek-Klondike and Dominion Creek tephras (ca 80 ka) exposed in west- Received in revised form central Yukon. These middens provide plant and insect macrofossil evidence for a steppe-tundra 8 January 2010 ecosystem during the Early Wisconsinan (MIS 4) glacial interval. Midden plant and insect macrofossil Accepted 21 April 2010 data are compared with those previously published for Late Wisconsinan middens dating to Available online 23 May 2010 w25e2914C ka BP (MIS 3/2) from the region. Although multivariate statistical comparisons suggest differences between the relative abundances of plant macrofossils, the co-occurrence of steppe-tundra plants and insects (e.g., Elymus trachycaulus, Kobresia myosuroides, Artemisia frigida, Phlox hoodii, Con- natichela artemisiae) provides evidence for successive reestablishment of the zonal steppe-tundra habitats during cold stages of the Late Pleistocene. Arctic ground squirrels were well adapted to the cold, arid climates, steppe-tundra vegetation and well-drained loessal soils that characterize cold stages of Late Pleistocene Beringia. These glacial conditions enabled arctic ground squirrel populations to expand their range to the interior regions of Alaska and Yukon, including the Klondike, where they are absent today. Arctic ground squirrels have endured numerous Quaternary climate oscillations by retracting populations to disjunct “interglacial refugia” during warm interglacial periods (e.g., south-facing steppe slopes, well-drained arctic and alpine tundra areas) and expanding their distribution across the mammoth-steppe biome during cold, arid glacial intervals. Crown Copyright Ó 2010 Published by Elsevier Ltd. All rights reserved. 1. Introduction terrestrial ecosystems for much of the Quaternary prior to the Last Glacial Maximum (LGM) or ecological responses to climate change The hallmark characteristic of Quaternary environmental change over successive glacialeinterglacial cycles. In reconstructing envi- is the cyclical oscillation between warm interglacial and cold glacial ronmental change over glacialeinterglacial time scales, it is impor- climate intervals, largely driven by orbital perturbations and their tant to recognize that for approximately 80% of the Quaternary, effects on solar insolation. Evidence for these climate fluctuations is global climates were colder than the present interglacial (Ehlers and recorded in d18O and other proxy data from deep sea marine sedi- Gibbard, 2004). As such, for many present-day organisms, in the ments (Martinson et al., 1987; Shackleton, 1987) and cores taken circumpolar region, glacial conditions, or at least conditions char- from the Greenland (Dansgaard et al., 1993) and Antarctic (EPICA, acterized by colder than present climates, can be considered to 2004) ice caps. However, long Quaternary terrestrial paleoecolog- represent the “norm” in terms of their typical ecological adaptations ical records are rare, especially in the northern hemisphere where (Rull, 2009). continental glacial advances have effectively removed much of the Unlike most of northern North America, vast areas of Alaska and sedimentary archive from previous intervals. Thus, there is limited adjacent Yukon (Eastern Beringia) escaped glaciation, and this understanding of the composition or function of high latitude region contains a terrestrial sedimentary archive that spans much of the Late Cenozoic (Hopkins et al., 1982; Froese et al., 2009). Since many of these deposits are older than the effective limits of w 14 * Corresponding author. Fax: þ1 867 667 5377. radiocarbon dating ( 50 000 C yr BP), development of a detailed E-mail address: [email protected] (G.D. Zazula). geochronology for this region has relied on other methods. In 0277-3791/$ e see front matter Crown Copyright Ó 2010 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.quascirev.2010.04.019 G.D. Zazula et al. / Quaternary Science Reviews 30 (2011) 2220e2237 2221 particular, much of Eastern Beringia is in the fall-out zone of distal Knowledge about Beringian biota from multiple intervals can help tephra from the Aleutian and WrangelleSt. Elias volcanic fields us understand the nature of northern biotic refugia and address (Preece et al., 1999, 2000; Begét, 2001; Westgate et al., 2008). how these ecosystems and organisms respond to climate change Tephra can be dated with direct (i.e. fission track, Westgate, 1989) over glacial-interglacial time scales. and indirect (i.e. luminscence or from associated radiocarbon ages, In this paper, we present plant and insect macrofossil data from Berger et al., 1996; Froese et al., 2002; Demuro et al., 2008) methods 64 fossil arctic ground squirrel (Spermophilus parryii) middens and correlated regionally by geochemical analyses. At least 70 (nests and caches) recovered in association with SCt-K/DCt (early tephras have been identified in Eastern Beringia thus far and have MIS 4 marker beds, ca 80 ka) from ice-rich loess exposures in the facilitated the study of regional Late Cenozoic environmental Klondike goldfields of west-central Yukon (Fig. 1). Pleistocene arctic change (Westgate et al., 1990; Begét, 2001; Westgate et al., 2005). ground squirrel middens have been recognized and examined from Frozen “muck” sediments are commonly exposed by placer gold Siberia (Gubin et al., 2001, 2003; Zanina, 2005), Alaska (Guthrie, mining in valleys of central Alaska and Yukon, yielding a wealth of 1990) and Yukon (Harington, 1984, 2003; Zazula et al., 2005, Pleistocene fossil biota (Guthrie, 1990; Péwé et al., 1997; Harington, 2006a, 2007) and consist of frozen amalgamations of plants 2003; Zazula et al., 2005, 2007). Muck consists of perennially remains that include both graminoid vegetative materials used by frozen, unconsolidated, organic-rich loess found commonly along arctic ground squirrels for nests, and seeds and fruits that were north and north-easterly facing sites and in the bottoms of narrow cached over the winter (Zazula, et al., 2005, 2006b). Plant and other valleys (Fraser and Burn, 1997; Kotler and Burn, 2000; Froese et al., biotic remains from the middens provide detailed, local paleoeco- 2009). Throughout the summer, new exposures of muck are logical records because present-day arctic ground squirrels are constantly created through natural melting and placer mining. known to forage in a relatively limited radius from their nests In the ice-rich loessal deposits of the Klondike goldfields in west- (Batzli and Sobaski, 1980; Zazula et al., 2006b). Although the central Yukon, Dawson tephra (Dt), dating to w25 30014CyrBP paleoecological data from midden samples are certainly biased (w30 000 cal. yr BP) serves as a stratigraphic marker for the onset towards preferred cache forage and nest building plant taxa (Zazula glacial conditions at the Marine Isotope Stage (MIS) 3/2 transition et al., 2006b), the wide range of species represented in previously (Froese et al., 2002, 2006, 2009; Zazula et al., 2006a), while Sheep analyzed fossil samples suggest that they adequately represent Creek tephra-Klondike (SCt-K) and Dominion Creek tephra (DCt), overall local plant community composition (Zazula et al., 2005, dating to 82 Æ 9 ka, mark the onset of the Early Wisconsinan glacial 2007). These midden data are compared with those from 48 interval of MIS 4 (Westgate et al., 2005, 2008). Reconstructing fossil middens which date to the Late Wisconsinan glacial interval paleoenvironments associated with these tephras has been the (MIS 3/2 transition) w25 and 29 ka14CyrBP(Zazula et al., 2007). focus of interdisciplinary research (Sanborn et al., 2006; Zazula Comparison of the Late Wisconsinan (MIS 3/2 transition) and Early et al., 2006a). Paleoenvironmental records for early MIS 2 (Late Wisconsinan (MIS 4) middens are used to examine vegetation, Wisconsinan) from the Klondike include fossil arctic ground squirrel arctic ground squirrel adaptations and the ecology of mammoth- middens (Zazula et al., 2005, 2007), peat and in situ vegetation steppe biome during two successive glacial intervals. (Zazula et al., 2003, 2006a; Froese et al., 2006) and loessal paleosols (Sanborn et al., 2006). Together, these data provide evidence for 2. Methods zonal cryoxerophilous steppe-tundra vegetation that occupied well-drained loessal soils during the onset of the last major Pleis- Fossil arctic ground squirrel middens (Figs. 2 and 3)were tocene cold stage. collected at exposures of Pleistocene ice-rich silt along Dominion Although many of the